The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the transmission ratio at a minimum transmission ratio.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid-operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to control the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
At a minimum transmission ratio (overdrive), as shown in FIG. 6, the ratio (P.sub.P /P.sub.S) of oil pressure P.sub.P of the servo device of the drive pulley to oil pressure P.sub.S for the driven pulley is higher than a value (.alpha.) necessary to keep the overdrive operation. Driving under such a high oil pressure causes an increase of wear of the belt and a delay in downshifting operation from the overdrive.
Japanese Patent Laid Open No. 59-217051 discloses a system for resolving such problems. In the prior art, the overdrive transmission ratio is set to a value lower than a minimum value which is minimum within a controllable range. Such an overdrive transmission ratio can be provided by a feedback control in which the overdrive ratio is set as a desired value. However, if the transmission ratio is controlled by the feedback controlling, hunting of the control system inevitably occurs, which swings the transmission ratio. The fluctuation of the transmission ratio at high vehicle speed causes a variation of engine speed which is unpleasant to the driver.